Hi...AND WELCOME!!!
Keep the little one warm and something fuzzy like a feather duster to snuggle into.
OK regardless of what species it is it is very important to empty the bird of feces either prior or after feeding. Have you noticed the little one pointing its rump or little tail up? If so what you do is take a kleenex, Q-tip, or your fingertip and massage the vent until it poops. In the nest the parent birds do this to help thge baby poop. The parent birds eat the feces, and many times it is fed back to the baby when they feed it food. You do not have to feed it back, I was just explaining what partents do. So Make sure that each time fed (either prior or after) the baby is stimulated to poop. I failed to do this on one, and I suspect I lost it from an intestinal impaction.
As to feeding....if you touch it on the side of the beak does it give a feeding responce? They will ususally flutter their little wings and their body looks like it is vibrating. You can chop up fruit into tiny sized peices and use a tweezers to place the fruit in the babies mouth when it begs/opens it's mouth for food. In addition use your commercial handfeeding formula, mixed with mashed or baby food fruits to syringe or feed from a tiny paddle or spoon.
Mousebirds do not have a crop, like parrots, which will hold quanties of food. The have an expandible esphogus, so they will only hold about 1cc or so of food at a time.
OK...PLEASE try and post a digital pix of your baby...TOP and SIDE VEIWS. I can get an idea of what species it is. And also time/amountsd to feed. I'm suspecting possibly a speckled, which I do not work with, but I do know there are some mousebird rehabbers in Africa....I can look up email info, if needed.
My other concern is cat saliva on the baby. This could be a problem. Do you have any access to penecillin...just in case, if needed for an infection?